Recording of sound



Dec. 6, 1938. E. w. KELLOGG RECORDING OF SOUND Filed July 11, 1932 2 Sheets-Sheet 1 INVENTOR EDWARD)W. BY h W K LLOGG.

ATTORNEY Dec; 6, 1938. E. w. KELLOGG RECORDING OF SOUND Filed July 11, 1932 2 Sheets-Sheet 2.

UUUUUUUU INVENTOR EDWARD W KELLOGG BY g W/ ATTORNEY Patented Dec 6, 1938 UNITED STATES 2,139,235 RECORDING OF SOUND Edward W. Kellogg, Moorestown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July-11, 1932, Serial No. 621,787

8 Claims.

This invention relates to the recording of sound on a light sensitive record, and has for its principal object the provision of an improved recording apparatus and method of operation 5 whereby background noise, such as that caused by dust or scratches on the clear part of the sound track, is avoided.

A further objectis the provision of a recording galvanometer by which the necessary func- 10 tions are performed in a simpler manner than has hitherto been possible.

Another object of my invention is the provision of a recording galvanometer wherein the vibrating mirror is caused to execute the sum of 15 two movements, one movement being in accordance with the sound vibrations to be recorded and the other movement being in accordance with the ehvelope of the rec-rd of the said sound vibrations.

Another objectpf my invention is to provide a recording galvanometer, wherein the electrical circuits through which the currents for producing the saidmovements are supplied, are separated.

25 Another object of my invention is to provide a recording galvanometer of the kind described, whose current requirements for the execution of the envelope movements, will be small enough to be operated by the anode current of one or 30 two moderately low voltage vacuum tubes.

, I shall, for convenience in the following discussion, refer to the current which causes the envelope movement, as the bias current, and the coil through which it flows as the bias coil or winding and the current which causes the audio frequency vibrations as the voice current, although it will be understood that the sound being recorded may be the human voice, or music, or any other sounds.

4:0 Various types of apparatus have been provided in the past for minimizing the background noise (commonly designated as ground noise) which results from dust, scratches and the like on the clear part of a sound record through 45 which light is applied to the light sensitive cell of a sound repr-oducer. As disclosed by United States Patent 1,855,197, a shutter actuated in accordance with the envelope of the sound record may be provided for shading so much of the 50 clear part of the record as is not required for proper reproduction of the sound. As indicated by Patents 1,853,812 and 1,854,159, both the audio impulses and the ground noise elimination current which varies in accordance with the en- 55 Vel-ope of the recorded impulses may be supplied to the single coil of arecordin'g galvanometer. In the latter apparatus, the median about which the recording beam oscillates at the recording surface is shifted so as to maintain the record peaks near the edge of the sound track irrespec- 5 tive of modulation of the audio impulses, thus reducing the clear part of the track to the minimum necessary for proper reproduction.

The recording optical systems in which the envelope biasing current has been supplied to the galvanometer, in the past have been so arranged that when the sound record amplitude is small the waves are recorded near one side of the sound track. This is not an advantageous condition and has led to the use of the shutter system already mentioned in spite of the requirement of extra apparatus. I prefer to use the optical system illustrated in Fig. 3 which makes a sound track illustrated in Fig. 4:, which affords the advantage of keeping the sound wave record in the middle of the track, but does not require a shutter. My invention, however, .is equally applicable to any system in which a biasing current is supplied to the galvanometer.

It has been found desirable to make sound recording galvonometers very low impedance, by employing only a small number of turns on the voice coil. This is in part for the reason that a low impedance coil can 'be made more compact and thereby placed closer to the working air gaps of the magnetic system, with the result that there is less leakage hurt and lower inductance than would be the case with a more bulky coil.

The low impedance winding calls for a comparatively large current at low voltage, and this requirement is readily met by supplying the voice current from a suitably designed transformer. For biasing purposes on the other hand, a transformer cannot be employed, since it must be possible to sustain any given uni-directional current for an indefinite period. The bias current must therefore be supplied directly from a rectifier or from an amplifier tube anode circuit, with either of which it is simpler and more eificient to obtain a small current at comparatively high voltage. 5 It is thus desirable that the biasing effect be obtained by sending a small current through a large number of turns of wire, or in other words that the biasing be accomplished by providing a separate coil wound for high impedance. Since leakage inductance is not prejudicial in the bias circuit, the extra bulk of the high impedance coil is not objectionable, and it may be placed behind or wound outside of the voice coil without detriment to its own function. The extra winding therefore does not interfere with placing the voice coil close to the air gaps. Another advantage of providing a separate bias winding is that it is unnecessary to employ any impedance devices to keep the bias current from being partially wasted by flowing through the voice coil feed circuit.

The invention will be better understood from the following description when considered in connection with the accompanying drawings, and its scope will be pointed out in the appended claims: I

Referring to the drawings,

Fig. 1 illustrates a two-coil recording galvanometer wherein the invention has been embodied,

Fig. 2 is a diagrammatic showing of a recording system in which this galvanometer is utilized,

Fig. 3 illustrates the manner in which the galvanometer is utilized in one type of recording optical system.

Fig. 4 illustrates the type of record produced by the recorder of Fig. 3,

Fig. 5 illustrates the use of the galvanometer in a different type of recording optical system, and

Fig. 6 illustrates the type of record produced by the recorder of Fig. 5.

The galvanometer of Fig. 1 includes a magnet I provided with pole pieces II and I2 and a vibratable reed I3 which is held between the pole pieces at one end, is surrounded at its center by an audio current or voice current coil I4 and by a noise elimination or bias coil I and is pivoted to a mirror I6 at its opposite end. The mirror I6 is mounted on a very small V block I9 which is constrained from lateral movement by a metal ribbon I! attached to a bifurcated support I8 which is mounted on the pole pieces II and I2 and functions to tension the ribbon I! so as to hold the V block I9 in contact with the knife edge at the end of the vibratable reed or armature.

The operation of the galvanometer will be readily understood. As the reed is vibrated between the pole pieces II and I2 due to variation of the audio current in the coil I4, the mirror is rotated to and fro in accordance with these impulses. To the vibratory motion of the mirror in response to the audio current is added a more gradual movement due to the biasing coil which is supplied with a rectified" current changing in value in accordance with the envelope of the record of the impulses.

As indicated in Fig. 2, current for operating the galvanometer is derived from a microphone 2D or other suitable sound pick-up device, is amplified by an amplifier 2I, and is supplied from this amplifier to the audio coil I4, and also through an amplifier, rectifier and filter, indicated generally by the reference numeral 22, to the bias or noise elimination coil I5. It will, of

, course, be understood that a mixer or gain control and other elements commonly forming a part of the sound recorder, but not forming a part of this invention, may be provided. As explained in a copending application of G. L. Dimmick, Serial No. 610,301, filed May 9, 1932, now Patent 2,074,049, issued March 16, 1937, the filter may be arranged to accentuate the effect of the higher frequency component current on the bias coil. The rectifier renders the bias or noise elimination coil current proportional to the envelope of the recorded impulses, thus reducing the clear part of the record to that essential for proper reproduction of the sound.

As indicated by Fig. 3, light from a source 23 may be applied to the mirror I6 through lenses 24 and 25 and a stop 26 provided with a triangular opening. An image of the triangular opening in stop 26 is produced in the plane of the slit 29 by the intervening lenses. Rotation of the mirror causes a raising or lowering of the triangular image or light spot, thereby illuminating more or less of the slit. The lens 30 produces on the film 21 an image of the slot 29 or so much of the slit as is illuminated. As the film ill is moved past the optical system, while the mirror vibrates, a strip of varying width is exposed on the film, and a record of the type illustrated by Fig. 4 is produced. It will be ob served that this record is relatively narrow where the modulation is low and increases in width at higher modulations or at greater intensity, this result being due to the noise elimination or bias coil I5 which raises the triangular light beam, with'respect to the slit in the member 29, as the amplitude of the impulses increase. Under these conditions the record is automatically maintained at a width which prevents overlapping of the peaks but does not separate these peaks by a distance greater than that required for proper reproduction of the sound.

The recorder illustrated by Fig. 5 produces a type of record similar to that produced by the apparatus disclosed by Patents 1,853,812 and 1,854,159 mentioned above. In this type of record the median about which the recording beam vibrates at the recording surface is shifted so as to maintain the record peaks near one edge of the track. This recorder includes a light source 3| from which light is projected on the mirror I6 through a lens 32, a rectangular stop 33 and a lens 34. The light beam is reflected from the mirror I6 to a record 35 through the lens 34, lenses 36 and 31, a light slit member 38 and objective 39. The operation of this system is in general similar to that shown in Fig. 3

except that in Fig.' 5 the mirror rotates abouta vertical axis, and a rectangular light spot, the image of the opening in stop 33, moves longitudinally across the slit stop 38. The cylindical lens 36 serves to increase the light through the slit. The galvanometer window lens 34 serves to image the mask 33 in the plane of the slit 38. The optical system of Fig. 3 may if desired be provided with a window lens corresponding in function and position to lens 34. The screen 40 is to facilitate observation of the movements of the light beam. As in the case of the recorder previously described, the bias or noise elimination coil functions to move the median about which the beam vibrates at the recording surface so as to leave transparent only so much of the record as is necessary for proper reproduction of the sound.

In the construction of the apparatus, it has been found advantageous to use an audio current coil I4 which has an impedance of only a few ohms and is located close to the armature and to the pole pieces between which the armature vibrates. Such a coil is readily matched with the remainder of the apparatus by means of a transformer or the like and has the important advantage that leakage reactance is reduced to a minimum.

The noise elimination or bias coil I5 is preferably provided with alarge number of turns and is supplied with current through a comparatively high resistance or reactance output circuit which prevents drainage of the audio frequency power by transformer action between coils l4 and I5 into the bias circuit. Due to the comparatively large number of turns in the bias coil, less current is required to produce the required biasing effect and the size of the rectifier may be reduced materially. Since the bias current is uni-directional or changes comparatively slowly in value, the bias coil may be placed at some distance from the armature without pro-' ducing excessive leakage reactance. Mounting it either behind or on the outer periphery of the audio current coil gives a compact and satisfactory structure. 7

Having thus described my invention, what I claim is:

1. The combination of a member movable at audio frequencies, a source of electrical impulses, a relatively low impedance coil arranged to control the movement of said member in accordance with said impulses, and means including a relatively high impedance coil arranged to cause said movement to be modified in accordance with the envelope of said impulses.

2.The combination of a mirror vibratable at audio frequencies, a source of electrical impulses, a relatively low impedance coil arranged to control the vibrations of said mirror in accordance with said impulses, and means including a relatively high impedance coil arranged to modify said vibrations in accordance with the envelope of said impulses.

3. The combination of a source of electrical impulses corresponding to sound impulses, means for producing a current which varies in accordance with the envelope of said impulses, a galvanometer including means for producing a magnetic field, an armature arranged to move in said field, a relatively low impedance coil connected to said source for controlling the movement of said armature in accordance with said impulses, and a relatively high impedance coil arranged to modify said movement in accordance with said envelope current.

pulses, means for producing a current which varies in accordance with the envelope of said impulses, a galvanometer including means for producing a magnetic field, an armature arranged to move in said field, a relatively low impedance coil connected to said source for controlling the movement of said armature in accordance with said impulses, and a relatively high impedance coil arranged to modify said movement in accordance with said envelope current.

5. The combination of means for producing a magnetic field, an armature arranged to vibrate at audio frequencies in said field, a reflecting element arranged to be moved by said armature, and separate relatively low and relatively high impedance coils arranged to control the movement of said armature.

6. The combination of means for producing a magnetic field, an armature arranged to vibrate at audio frequencies in said field, a reflecting element arranged to be moved by said armature, a relatively low impedance coil mounted in close proximity to said armature for controlling its movement, and a relatively high impedance coil surrounding said relatively low impedance coil v for modifying said movement.

'7. The combination of a movable member, a source of electrical impulses of audio frequencies, means for producing a magnetic field, a low impedance coil arranged in proximity to said field for controlling the movement of said member in accordance with said impulses, and means including a high impedance coil spaced from said field for causing said movement to be modified in accordance with the envelope of said impulses.

8. The combination of means for producing a magnetic field, an armature arranged to vibrate at audio frequencies in said field, means including a vacuum tube provided with an anode circuit, and separate relatively low and relatively high impedance coils arranged to be energized from said circuit for controlling the movement of said armature.

EDWARD W. KELLOGG. 

